1. Technical Field of the Invention
The present invention relates to electric motors and, more particularly, to an electric motor with low cogging torque for use in an automobile and a truck or the like and a method of determining a stator pole geometry for controlling cogging torque in the electric motor.
2. Description of the Related Art
As technologies to reduce cogging torque of electric motors in general practice, skewing technique has been explored to reduce cogging torque. Skewing technique features to form stator poles in a skewed pattern such that a stator has rotational positions deviated along an axial direction of a rotor. With such a structure, the stator poles are deviated at an electric angle of 180 degrees to allow the stator to have the same amount of air gap in any rotational positions. In theory, this enables the stator to have a zeroed difference in magnetic reluctance along a circumferential inner periphery of the stator, causing no cogging torque to occur. However, with the skew angle set at too large a value, a drop occurs in utilization efficiency of a magnetic flux of the rotor, resulting in a decrease in output torque. In the case of the stator deviated by 180 degrees, the electric motor generates torque half that of an electric motor with no deviation of the stator poles. Therefore, in actual practice, no attempt is made to deviate the stator poles at the electric angle of 180 degrees; the skew angle is determined so as to cancel a frequency component with maximal cogging torque caused by the stator with no skewing on the stator poles. In such a case, cogging torque cannot be completely removed.
Further, another attempt has been made to reduce cogging torque by additionally forming auxiliary notches as disclosed in T.IEE Japan, Vol. 122-D, No. 4, 2002 on page 338 to 345 entitled “Cogging Torque Reduction in Permanent Magnet Brushless Motors”. With such a related art, the auxiliary notches are additionally inserted to positions to cancel variation in magnetic reluctance that occurs in an air gap portion between stator cores.
By the way, with the related art technique of employing added notches as disclosed in the above literature, magnetic reluctances occurring in the air gap segments between the stator cores and the added notches unequally vary with the resultant occurrence of issues wherein the added notches are effective to reduce cogging torque to some extent but have no remarkable effect of completely minimizing cogging torque.